Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

Abstract

Coherent phonons can be launched in materials upon localized pulsed optical excitation, and be subsequently followed in time-domain, with a sub-picosecond resolution, using a time-delayed pulsed probe. This technique yields characterization of mechanical, optical, and electronic properties at the nanoscale, and is taken advantage of for investigations in material science, physics, chemistry, and biology. Here we review the use of this experimental method applied to the emerging field of homo- and heterostructures of van der Waals materials. Their unique structure corresponding to non-covalently stacked atomically thin layers allows for the study of original structural configurations, down to one-atom-thin films free of interface defect. The generation and relaxation of coherent optical phonons, as well as propagative and resonant breathing acoustic phonons, are comprehensively discussed. This approach opens new avenues for the in situ characterization of these novel materials, the observation and modulation of exotic phenomena, and advances in the field of acoustics microscopy.

Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in α‐ and β‐In2Se3

“…The principles of the PU technique are described elsewhere [ 29,30 ] and the details of the PU experiments applied to other vdW layers can be found in earlier publications. [ 32–39 ]…”

“…The principles of the PU technique are described elsewhere [29,30] and the details of the PU experiments applied to other vdW layers can be found in earlier publications. [32][33][34][35][36][37][38][39] We use three PU methods for extracting the sound velocity s from the temporal dependence of the pump-probe signal. The schematics of these methods are illustrated in the upper panels of Figure 2.…”

“…In thin flakes (d < 100 nm), R(t) shows oscillations due to phonon quantization, as reported for various vdW nanolayers. [32][33][34][35][36][37][38][39] In this case, the fundamental frequency of the flake vibrations is given by f = s/2d or f = s/4d depending on the elastic properties of the interface. [36,38] Then the value of s is obtained from the measurements of f or its higher harmonics.…”

Anisotropic Generation and Detection of Coherent Ag Phonons in Black Phosphorus

“…While inelastic light scattering in the frequency domain is measured via Raman scattering, electron-phonon interactions can be investigated in the time domain by generating and detecting coherent phonons (CPs) with ultrashort optical pulses [28][29][30][31][32]. Upon electronic perturbation by an ultrashort pulse, the lattice is driven to macroscopic motions, thereby generating CPs of specific modes.…”

3D Hypersound Microscopy of Van der Waals Heterostructures

“…Nanoindentation 14 and atomic force microscopy 15 do not provide a comprehensive information on mechanical properties of multilayer structure as they require subtraction of huge contribution related to the substrate. Picosecond ultrasonics or picoacoustics appears to be much more effective tool to access the rigidity of the flakes and interfaces in the case of thin multilayer structures 16 .…”